Our specific aim is to go further with studies originally proposed, whereby we ask: a) What are the relative contributions of the germ line, combinatorial, and somatic variation mechanisms to the functional repertoire of antibody molecules? b) Is somatic diversification a random process happening at the base somatic mutation rate or is it a consequence a V-gene specific mechanism? c) At what stage of B cell differentiation does somatic mutation take place? The methods by which we will approach these questions are purely analytical. We will compare expressed VH gene sequences at different stages of the immune response to the hapten (4-hydroxy-3-nitrophenyl)acetyl. Furthermore these sequences will be compared to the germ line VH gene repertoire. In an attempt to approach the question of control of VH gene expression, we will use a unique cell model to test the hypothesis that functionally rearranged immunoglobulin heavy chain genes or proteins directly inhibit transcription of immunoglobulin genes located on the other chromosome.